In the prior art the utility of a laminated sheet constructed of a foamed thermoplastic core sandwiched between two sheets of covering material has long been recognized. For example, in U.S. Pat. No. 2,770,406 issued to Gardiner Lane on Nov. 13, 1956, a carton is disclosed which is made from a laminate with a polystyrene foam core sandwiched between two sheets of kraft paper. Such a laminate is adaptable for replacing corrugated paperboard or wood in many carton and crate applications and is particularly useful where insulation is required for frozen items such as ice cream and frozen meat and vegtables or as a shipping carton for chilled food and produce. In such prior art applications the density of the polystyrene foam may vary between about two to about 10 pounds per cubic foot with a commonly preferred prior art density of about 3 pounds per cubic foot. If the density approaches ten pounds per cubic foot an unnecessary expense for the plastic material will be involved; but, on the other hand, if the expansion of the foam is carried too far the gas bubbles formed therein can easily become much too large resulting in a foam product which when cooled will tend to be too brittle and would crack when any pressure is applied thereto. Thus, according to the disclosure in the aforementioned Lane patent, it has been found that a material having a density in the neighborhood of 3 pounds per cubic foot appeared to be the most suitable for lamination between sheets of kraft paper. Furthermore, the Lane patent suggests as one particularly convenient method of forming such a laminate the placing of a sheet of polystyrene foam containing a foaming agent between two of the kraft paper sheets so that if the heating and foaming take place in the polystyrene under some pressure against the heated kraft sheets, the sandwiched parts may be made to adhere together without interposing any adhesive material.
Another prior art method of forming such a laminate is found in U.S. Pat. No. 3,062,698 which issued on Nov. 6, 1966 to A. A. Aykanian. In the Aykanian patent a method is disclosed for continuously making a laminate of polystyrene foam and kraft paper wherein the only heat applied to the previously formed polystyrene foam is through the kraft paper so that the foam is not heated or softened prior to its contact with the paper and the transfer of heat is through the paper to the foam.
In still another prior art process which is described in U.S. Pat. No. 3,823,047 which issued on July 9, 1974 to Roberto Colombo, all of the bonding heat is accumulated in the kraft paper sheet which is heated to a temperature exceeding the melting point of the resin prior to its contact with the styrene foam which is at ambient temperature.
It is a primary object of the present invention to provide a process for manufacturing a thermoplastic foam laminate which is an improvement over the prior art processes described above.
It is another object of the present invention to provide a process in which foam of lower density than that used in the prior art can be employed.
It is another object of the present invention to provide a process for making a laminated foam article whereby the probability of cracking the cells in the foam laminate is lessened.
Still another object of the present invention is to make a laminated article having a foamed core sandwiched between covering sheets which exhibits superior adhesion of the covering sheets to the foamed core and is flatter and more uniform in thickness than prior art laminates.
All the foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art from the following Summary of Invention and the Detailed Description of the Invention which follow.